We have isolated cellular mutants of mouse cells which are resistant to the transforming activity of some specific onc genes including ras, fes and, possibly, src. Other onc genes including mos, fms and sis appeared to be able to express their transforming activities in these mutants, as assessed either by direct infection with various sarcoma viruses or by cell fusion to cell lines transformed by various onc genes. To identify the genetic lesion(s) responsible for the behaviour of the mutants, we conduced two lines of experiments whose results were as follows. 1) 86Rbt (a tracer for Kt ion)-uptake of these mutants showed altered reresponse to treatment with a specific inhibitor of Nat/Kt-ATPase, onabain, as compared to those of a normal and a ras-transformed mouse 3T3 cells. This suggests that some alterations in membrane structure might be responsible for the mutant phenotypes. 2) The mutant phenotypes could be transfered into fresh cells by DNA-transfection technique. This technique should provide a basis for directly identifying the gene(s) responsible for the mutant phenotypes, and for studying the interaction of these genes with certain onc genes.